In general, methods for forming multilayer coating films on vehicle bodies are carried out by forming an electrodeposition film on an object to be coated, curing this film by heating and then forming a multilayer coating film comprising an intermediate coating film, abase coating film and a clear coating film. In addition, water-based coating materials have come to be used as intermediate coating materials and base coating materials in order to reduce the quantities of volatile organic components (VOCs) used.
Furthermore, due to demands for reduced energy consumption in recent years, base coating films and clear coating films are formed on preheated intermediate coating films formed on electrodeposition film without curing the intermediate coating film by heating, and these three films are then simultaneously cured by heating. This is a so-called 3 coat 1 bake (3C1B) method, and is a method that has come to be used as a method for forming multilayer coating films.
In order to form a multilayer coating film having excellent surface smoothness by effectively preventing mixing of an intermediate coating film and a base coating film in methods for forming multilayer coating films using this type of 3C1B method, it is effective to reduce the water absorption and water elution of an intermediate coating film formed from a water-based intermediate coating composition and incorporate an acrylic resin emulsion having a specific glass transition temperature, acid value and hydroxyl group value, a urethane resin emulsion having a specific acid value and a curing agent (see Japanese Unexamined Patent Application Publication No. 2004-358462).
In addition, in order to obtain a multilayer coating film having excellent appearance and water resistance by using a multilayer coating film formation method in which a first coating material, a second coating material and a clear coating material are coated using a 3C1B method, it is effective to incorporate specific quantities of an emulsion resin obtained by emulsion polymerizing a mixture of specific vinyl monomers, an amide group-containing water-soluble acrylic resin, a urethane resin emulsion and a curing agent as a water-based coating composition for forming the second coating material (see Japanese Unexamined Patent Application Publication No. 2007-297545). In addition, Patent Document 2 indicates that it is possible to impart a water-based coating composition with thixotropic properties by imparting the above-mentioned emulsion resin with a core/shell structure.
Furthermore, in order to form a multilayer coating film having excellent smoothness, vividness and chip resistance by using a multilayer coating film formation method in which a first water-based color coating material, a second water-based color coating material and a clear coating material are coated using a 3C1B method, it is effective to incorporate an acrylic resin, a curing agent and a urethane resin emulsion as the first water-based color coating material and also for a first color coating film formed from this first water-based color coating material to have a specific water swelling ratio and organic solvent swelling ratio (see WO 2010/082607). In addition, WO 2010/082607 indicates that the above-mentioned urethane resin emulsion is produced using a polyisocyanate component and a polyol component as raw materials and that a compound containing a hydrocarbon group having six or more carbon atoms is preferred as a raw material compound for this polyol component.
However, in cases where preliminary heating is not carried out after the first coating film is formed in a multilayer coating film formation method using a 3C1B method and the coating compositions disclosed in Japanese Unexamined Patent Application Publication No. 2004-358462, Japanese Unexamined Patent Application Publication No. 2007-297545, and WO 2010/082607 are used, layer mixing occurs between the first coating film and the second coating film and it is not possible to obtain a coating film having a good appearance.